There is a known art related to a sensor module, including a humidity sensor and a controller for scanning a proper measured value, that varies according to temperature change, from a detected value produced by the humidity sensor, based on a data table prepared with a setting value of temperature varied by a physical quantity changing device. However, in the art, when detecting humidity around an external apparatus such as an image forming apparatus, to use the sensor module including the controller results in a higher cost, thereby further increasing the cost for mounting the physical quantity changing device for controlling the variation of detected values.